The present invention relates to a system for transmitting data over power-carrying wires and, more particularly, to a system, for carrying commands and control signals over power-carrying wires to lamp controllers, that is relatively immune to disturbances prevalent on such wires.
In a controlled lighting system, such as deployed particularly in commercial and industrial premises, there are typically a plurality of lamp clusters, each controlled by a controller, and one or more command stations, from which various lighting functions may be controlled by users. Usually the lamps are of the fluorescent type and a controller includes an electronic ballastxe2x80x94to stabilize the light. An important lighting function is dimming, that isxe2x80x94controlling the light level of each lamp cluster. Commands for changing the light level or for setting it at a desired value are usually issued by the user by means of a command station. Such commands may also be issued by a computer or by some appliance control center (such as deployed in a home). In some systems there are also deployed light level detectorsxe2x80x94for monitoring the actual light level at certain points and issuing feedback control signals to the controllers, so as to maintain some predetermined light level.
All such commandxe2x80x94and control signals must be transmitted from their respective sources to one or more of the lamp controllers. Such transmission may be effected either by wireless means, or over a pair of wires, dedicated to the purpose, or over wires that primarily serve for the transmission of power and that usually must also be connected to the controllers and to the various command-issuing units. Wireless transmission requires relatively expensive transmitting and receiving equipment for reliable operation, especially for systems with large spatial extent. Transmission over dedicated wires requires that such wires be specially installed; such installation may be relatively expensive, especially in older premises. On the other hand, power mains are ubiquitous in most buildings and easily accessible from all units; moreover, in many lighting systems full power wiring may already exist prior to the installation of the control features and components and thus represents a readily available conduit for the control signals.
The present invention is primarily concerned with method and apparatus for transmitting lighting control signals over power-carrying wires. However, while the invention will be described in terms of an embodiment within a lighting control system, it should be understood to apply, with obvious modifications, also to other systems that require transmission of data or control signals over power-carrying wires, such as heating- and cooling systems, industrial production lines or home appliance control systems.
From the point-of-view of signal transmission, power lines are electrically very noisy, that isxe2x80x94they carry, in addition to the power (at a frequency of 50 or 60 Hertz, and its harmonics), randomly varying voltages of relatively large magnitude. Part of such noise comes over the mains from outside the controlled system and part of it may be generated within the system xe2x80x94e.g., by switching power supplies, electronic ballasts, motors, lamp instabilities etc. The time variation of the noise voltages may be categorized into three groups: (1) continuous wide-band random variation (akin to xe2x80x9cwhitexe2x80x9d noise), (2) continuous narrow-band disturbances (about distinct frequencies) and (3) randomly occurring narrow pulses. In order to reliably transmit data in the presence of such noise and disturbances, the transmission method (that is xe2x80x94the characterization of the signals and the manner in which data is encoded onto them and decoded from them) must be optimally designed. Moreover, the impedance of power lines is generally not constant, but rather frequency dependent and variable with time.
Another important characteristic of power-line transmission systems, especially when serving power control signals, is the relatively low rate at which data need generaly be transmitted. Effective data rates of less than a thousand bits per second are usually sufficient. A third, related, characteristic is that the controlled system, with its power wiring, has a limited spatial extent and that the power transmission medium may generally be regarded as dedicated to the system.
An important requirement from most practical power-line transmission systems is that the necessary hardware components, especially those associated with the controlled devices (and thus usually functioning as signal decoders) carry a low cost. This is due to the fact that most of such devices, such as motors, lamp ballasts and regulators, themselves carry relatively low costs.
Most methods of prior art for transmission over power lines have been largely based on methods for transmission over other media, such as wire pairs, coaxial cables and wireless transmission channels. Such methods, well known in the general fields of communications and data transmission, are optimized for transmission conditions on such other media and particularlyxe2x80x94for their noise characteristics. The latter are, in most cases, of the wide band type, but in certain systems, one of the other two types mentioned above (continuous distinct frequencies or impulses) may be predominant and the transmission methods optimized for the relevant one of them. Generally, however, few of the known transmission methods is specifically designed to overcome all three mentioned types of disturbances (noise), characteristic to power lines, simultaneously.
Moreover, traditional transmission systems are designed for relatively high rates of data. Although various spectrum-spreading methods are utilized, the available bandwidth generally limits the spread ratio. On the other hand, as has been mentioned hereabove, power system control signal are of relatively low rate and thus it stands to reason to simply modify a known method so as to spread the spectrum widely enough to reliably overcome all noise types prevalent on power lines. In practice, this proves to yield insufficient reliability vs. the achieved transmission rate.
Finally, the hardware components required by systems based on current transmission methods are relatively expensive. Even components currently offered commercially, specifically for power-line communication, are too expensive for most practical lighting systems. The following examples may serve as an indication of typical costs: In a xe2x80x9cX-10xe2x80x9d system, a remote unit (i.e. one attached to a controlled device) costs about $20. A power-line modem is offered by Echelon Corp. at about $120 and by Intellon Corp.xe2x80x94for $100-200. By contrast, the present invention aims at enabling a remote reception component to sell for between $1 and $10.
U.S. Pat. No. 5,579,335 discloses apparatus and method for decoding signals transmitted over power lines, using band-splitting filters, delay-line correlators, which operate on two separate frequency bands, and further signal processors. The method enables transmission rates in the order of 100 kb/s (which is much higher than required in practical control systems), but the apparatus is inherently expensive.
U.S. Pat. No. 5,448,593 discloses a system for network communication over power lines, which uses two-frequency FSK modulation and an error coding system to control reception quality; upon analyzing errors in the received signal, receiver gain is modified or different frequency pairs are selected for keying or the transmitted bit rate is adjusted. The transceiver apparatus for such a system is, again, inherently expensive and has additional drawbacks in that it (a) assumes stationary noise conditions over a certain period, which is not always the case (especially with impulse-type noise), and (b) requires a reverse transmission path (and thusxe2x80x94additional transmission apparatus), which otherwise is not necessary and further increases system cost.
U.S. Pat. No. 5,448,593 discloses a system for transmitting and detecting signals over power lines to control the dimming of fluorescent lamps. It uses binary FSK and doubles each word transmitted to reduce errors. The detection apparatus is based on analog circuitry, which inherently includes many discrete components and thus is disadvantageously expensive. There is thus a widely recognized need for, and it would be highly advantageous to have, a method and corresponding apparatus for transmitting data over power lines that will be characterized by an acceptable transmission rate, high immunity to all types of disturbances prevalent on power lines and low cost.
The present invention successfully addresses the shortcomings of the presently known configurations by providing apparatus and method for communicating commands and data over power lines within a lighting control system, and more generally within a power control system, that is relatively inexpensive, yet wherein the data and commands are received with high reliability in the presence of all disturbances and noise commonly present on power lines.
The present invention involves a novel method for encoding the commands and data and for structuring the transmitted signal, as well as a novel architecture of a receiver for reliably and inexpensively detecting and decoding the data carried by a received signal.
More specifically, the encoding method of the present invention calls for structuring the transmitted signal as consecutive time slots, preferably in synchronism with the power frequency xe2x80x94typically one slot per power period. Each slot carries one symbol, selectable from a set of M symbols. Within each time slot there are consecutively transmitted N bursts of carrier frequencies. The frequency of any burst is selectable from a set of M mutually orthogonal frequencies. Typically all N bursts within any one time slot have mutually different frequencies. A unique combination of frequencies for N successive bursts is assigned to each of the M possible symbol values.
In a lighting control system, symbols typically carry commands and data. A command is preferably encoded onto a single symbol, there being Mxe2x88x922 possible different commands, such as xe2x80x9cincrease lightxe2x80x9d and xe2x80x9cdecrease lightxe2x80x9d, which are encoded onto Mxe2x88x922 corresponding symbol values. One symbol value is reserved for denoting xe2x80x9cidlexe2x80x9d condition (i.e. no commands or data transmitted) and one symbol value denotes xe2x80x9cbegin data stringxe2x80x9d. Data are encoded onto a string of successive symbols of fixed length, using any known encoding method, possibly including error-detection or error-correction codes, though the latter is generally not necessary when using the disclosed method.
The receiving apparatus, according to the preferred embodiment, consists of a front end, a set of M synchronous detectors which are implemented within a gate array, and a general-purpose microprocessor which is programmed to process detected values so as to identify symbol values and to decode the commands and data. The front end includes an amplifier, a broad band-pass filter and a signal binarizer. Each synchronous detector consists of a pair of correlators, which are fed the binarized input signal and a binary reference signal of a specific frequency (being a unique one of the aforementioned set of M frequencies), and an adder that receives the outputs of both correlators; the reference signal is applied to the two correlators in mutual phase quadrature. Each correlator consists of a XOR gate, followed by an up-down counter and an absolute-value device.
The received signal, after being filtered and binarized in the front-end, is applied to each of the 2M XOR gates, the other input of which is fed with the reference signal. The output of the XOR gate is applied as a gating signal to the up-down counter, which is also fed clock pulses. The counter counts clock pulses up or down according to the output of the XOR gate. At the end of each burst period the count is read out. If the frequency of the burst in the received signal is equal to the reference frequency, the count will generally have a finite value, proportional to the phase between them; if the two frequencies are not equal, the count will be essentially zero. In each of the M detectors, the absolute values of the two counts obtained from the pair of correlators, after each burst period, are added together in the adder and thus represent the phase-independent correlation value between the received burst and the specific frequency of the detector.
The M correlation values are fed to the microprocessor after each burst period and stored there. At the end of each symbol period (the aforementioned time slot), i.e. after each group of N bursts, groups of N stored correlation values are formed according to the association of corresponding frequencies with each of the M symbols (as has been assigned for encoding) and the values within each group are added together. The symbol associated with the highest sum is identified as the detected symbol. Symbols and sequences of symbols are thereafter decoded, by largely conventional methods, to obtain the carried commands and data.
It is noted that the fast digital operations are carried out within the Gate Array, whereas the slower operations are carried out within the microprocessorxe2x80x94which is an efficient and economic way of utilizing their capabilities. Moreover, the microprocessor may concurrently serve other control functions within the lighting- or power control system and the Gate Array may have other sections assigned to other fast control processes.
According to the present invention there is provided a method for transmitting data over power lines, the data being represented by a series of symbols, each symbol having one of a finite number of possible values, the method comprising encoding the series of symbols into a corresponding sequence of groups, each group consisting of N successive bursts of alternating signal, whereby each burst is assigned one of M possible frequencies, no two of said bursts in any one of said groups are assigned the same frequency and there is unique correspondence between the value of any symbol and the frequency of any one of said bursts within the corresponding group.
According to further features of the invention, the method further comprises decoding a group of said successive bursts, upon reception, whereby the decoding includes:
Providing M reference signals, each having a unique one of said M frequencies, and correlating each of said bursts with said M reference signals, resulting in M correlation values for each burst;
for each possible symbol value, selecting for each burst the correlation value that corresponds to the frequency assigned to that symbol value and adding all selected correlation values together, to yield a symbol score; and
selecting the highest symbol score to indicate the decoded symbol value.
According to further features in preferred embodiments of the invention described below, the correlation, with respect to each of said M reference signals, includes:
providing the reference signal as a binary valued reference signal;
converting the received signal into a binary valued received signal;
performing a XOR operation between the binary valued reference signal and the binary valued received signal;
providing a continuous train of clock pulses; and
counting said clock pulses up or down according to the results of said XOR operation.
Preferably the reference signal is provided as a pair of binary valued reference signals, each having the corresponding frequency and being in mutual phase quadrature and the XOR operation is performed between the binary valued received signal and each of the two binary valued reference signals.
According to still further features in the described preferred embodiments, said groups are timed in synchronism with the power frequency or any of its harmonics and all of said reference signals and the frequencies of all of said bursts are synchronized to a common reference frequency, which preferably is the power frequency.
According to the present invention there is also provided apparatus for decoding a signal received over a power line, the signal having been transmitted as a sequence of groups, each group encoding a data-carrying symbol, having one of a finite number of possible values, and consisting of N successive bursts of alternating signal, whereby each burst has been assigned one of M possible frequencies, the apparatus comprising:
at least one signal generator operative to produce a reference signal, or a pair of reference signals, which are in mutual phase quadrature, for each of the M frequencies;
at least one correlator for each of the M frequencies, operative to correlate the received signal with a corresponding reference signal, and to thus yield a correlation value at the end of each burst;
an adder for each possible symbol value, operative to add corresponding correlation values from a plurality of correlators, obtained at a plurality of bursts, and to thus yield a symbol score; and
a selection logic, operative to select from among the outputs of said adders the one with the highest symbol score.
According to further features of the invention, the apparatus further comprises a circuit to convert the received signal into a binary received signal, and a clock pulses generator and said at least one reference signal generator is operative to generate a binary reference signal; and said at least one correlator includes:
a XOR gate, receptive to said binary received signal and a binary reference signal;
an up-down counter, operative to count pulses, obtained from said clock pulses generator, up or down according to the output of said XOR gate;
According to still further features in the described preferred embodiments, the frequency of each of said reference signals is synchronous with the respective one of the assigned M frequencies and the apparatus further comprises a gate array, which includes the correlators and a phase-locked loop.
According to another embodiment of the invention, there is provided a lighting control system, for controlling lamps by means of signals carrying encoded control commands that are transmitted over a power line, the system having the features of the apparatus recited hereabove.
According to further features of the embodiment of the system, the control commands are classifiable into at least two classes, one class being a direct command and another class being a data message, wherein a direct command is encoded as a single symbol and a data message is encoded as a sequence of two or more successive symbols.